从对象数组中获取对象时，mongodb 中的查找不起作用

Question

Wishlist product

My code

Getting Wishlist Blanked. please help me why wishlist array shows no data

Answer 1

productId in the wishList is a String , while the _id field in product is an ObjectId .

You can change the type of the productId in the wishList or you can use $lookup with let and pipeline to project the _id in product as a String value before comparing it to wishList.productId , like this:

{
    "$lookup":
    {
        "from": "products",
        "let":
        {
            "productId": "$wishList.productId"
        },
        "pipeline":
        [
            {
                "$addFields":
                {
                    "idAsString":
                    {
                        "$toString": "$_id"
                    }
                }
            },
            {
                "$match":
                {
                    "$expr":
                    {
                        "$eq":
                        [
                            "$idAsString",
                            "$$productId"
                        ]
                    }
                }
            }
        ]
    }
}

I haven't tested the query, since you didn't provide an easy way to reproduce the error, as mentioned by toyota Supra in the comment to your question, but it should be something along these lines.